1. Field of the Invention
The present invention relates to a heat resistant alloy member using heat resistant alloy.
2. Background Art
For example, when a large-sized cast member such as a casing for a steam turbine is manufactured, it cannot be easily cooled because of its size and is kept at high temperature for a long time. Because the cooling speed is slow, the grain size especially in a thick portion is increased. When grains are coarse, the creep characteristics (creep strain rate) tend to be structurally favorable. However, the fatigue characteristics may be reduced.
For example, a cast member is used for the casing for the steam turbine, and the thickness of it may be between several tens and hundreds of mm when it is the large-sized (around 10 tons). In such a thick portion, the cooling speed is considerably reduced and grains tend to be coarse. In the casing, a portion where an upper half portion and a lower half portion are connected to each other with bolts is the thickest, and the edges of the bolt holes are prone to cracks due to fatigue. The cast member is also used for a valve for the steam turbine, the similar situation can occur when it is large-sized (several tons or more).
For example, in a combustor liner for a gas turbine, a number of micro cracks may occur by start or stop of operation. It is because the heat stress is generated by heat cycles in which high-temperature and low-temperature are alternately repeated and thus the low cycle fatigue is applied on a material.
A portion exposed to high-temperature steam or combustion gas needs to have the strength at high temperature. Accordingly, a material for such a portion is required to have heat resistance (high temperature strength). From this point of view, the fact that grains are large as in the casing for the steam turbine contributes to improvement of the creep strength at high temperature and reduction of the creep strain rate. However, the fatigue characteristics are reduced when grains are coarse. Furthermore, because the gas turbine often starts and stops, fatigue cracks may occur and expand on its surface by long-term use.
One conventionally known method of improving fatigue characteristics is to make the grains of a material finer (Patent Document 1: JP Patent No. 3229556). To improve the fatigue characteristics, friction stir processing is performed on the surface of an Al cast member for property modification according to Patent Document 1. However, Al alloy does not have the strength at high temperature, and accordingly, it cannot have the high creep characteristics like a high-temperature material used in a high-temperature portion of a thermal power plant.
When cracks occur in the individual members due to fatigue, repair welding is usually carried out. However, weld cracks may occur by the repair welding and thus the steps for repair welding may be increased. When the heat resistant alloy member is a forged member, the strength at a Heat Affected Zone (HAZ) or weld metal portion may be reduced in a weld portion as compared to a base material. Thus, the fatigue strength may be reduced as compared to the base material.
The grain size can be reduced by heat treatment, for example, in ferrite steel. In austenitic steel or Ni-based alloy, however, the grain size cannot be reduced by the heat treatment process. Furthermore, the grain size in a large-sized product or a product having a complicated shape cannot be easily reduced by restriction of power of a forging machine.
In recent years, it is desired that a thermal power plant reaches to high temperature, and accordingly, it is suggested that austenitic steel or Ni-based alloy is used for a high-temperature member. However, when such a high-temperature member is used, the fatigue characteristics may be reduced in a case where a large-sized structural member, which is required to have heat resistance such as a high temperature member of the thermal power plant, is manufactured, or cracks are repaired. Consequently, the reliability of the large-sized structural member may be reduced.
An object of the present invention is to provide a heat resistant alloy member that maintains the creep strength and improves the fatigue characteristics (suppressing the occurrence of cracks and their progress) of a large-sized heat resistant alloy material that tends to include coarse grains, and a method for manufacturing the same.
Another object of the present invention is to provide a method for repairing a heat resistant alloy member that prevents reduction of fatigue characteristics caused by repair of cracks that have occurred on a surface of the heat resistant alloy member.